Efficiently combining multiple taps of an optical filter

ABSTRACT

An optical filter comprises an array of waveguides fabricated on an optical integrated circuit (PIC). The array comprises individual waveguides, each of which receive light inputs, e.g., individual taps of a multi-tap optical filter used in an interference cancellation circuit. Typically, the output(s) of the individual waveguides are located at an exit (edge) of the PIC. At least one second waveguide in the array is patterned on the PIC in a converged configuration such that the light transiting these waveguides co-propagates and interacts across given portions of the respective waveguides before exiting the waveguide array along a common facet, thereby generating or inhibiting one of intermodulation products, and harmonics. This structural configuration enables the generation of various modes of transmission at the PIC exit, enabling more efficient transfer of the energy, e.g., to an associated photodetector (PD) that provides conversion of the energy to the RF domain.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. § 119 to U.S. patent application Ser. No. 16/511,715, filed Jul.15, 2019, and titled “Efficiently Combining Multiple Taps of an OpticalFilter,” the contents of which are incorporated herein by reference inits entirety.

TECHNICAL FIELD

This application relates generally to photonics and, in particular, tophotonic-based optical filtering.

BACKGROUND

Light-enabled technologies enable creation of faster, smaller,lower-powered components for interference-free device connectivity.These technologies replace conventional microelectronics with photonicprocessors that provide microphotonics-based real time signal processingto enable low interference high-bandwidth communications. Conventionalphotonic architectures include optical filter assemblies to facilitateinterference cancellation. In particular, when implementing opticalfilters in a photonics integrated circuit (PIC), light is split intomultiple paths and manipulated. These multiple paths of light are thencombined to obtain a filter output.

However, conventional methods of combining light paths, are large,bulky, or lossy and may be difficult to incorporate into small formfactor electronic devices.

BRIEF SUMMARY

An optical element comprises an array of waveguides fabricated on anoptical integrated circuit. The array comprises individual waveguides,each of which receive light inputs, e.g., individual taps of a multi-tapoptical filter used in an interference cancellation circuit. Eachindividual waveguide comprises an inlet that receives light, and anoutlet. Typically, the output(s) of the individual waveguides arelocated at an exit (edge) of a photonic integrated circuit (PIC) and, inparticular, along a common facet. In one embodiment, at least one secondwaveguide in the array is fabricated on the PIC in a tapered or“converging” configuration such that, relative to a first waveguide, thelight transiting these waveguides co-propagates and interacts acrossgiven portions of the respective waveguides before exiting the waveguidearray along the common facet, thereby generating or inhibiting one of:intermodulation products, and harmonics. In particular, this structuralconfiguration enables the generation of various useful modes oftransmission (e.g., supermode, and multi-mode) at the PIC exit, enablingmore efficient transfer of the energy, e.g., to an associatedphotodetector (PD) that provides conversion of the energy to the RFdomain.

The number of waveguides (taps) in the filter structure may vary, as cantheir shape(s) and relative spacing. In another aspect, the waveguidesmay be further separated by isolation elements to limit intermodulationproducts. Further, individual waveguides may include associated phaseshifter elements at their leading ends (i.e. their inlets) for furthersignal-shaping.

The PIC-based optical element may be used in association with any RFcomponent. In one embodiment as mentioned above, the optical filter isused in an interference cancellation circuit. Other applications of theoptical element herein include, without limitation, an RF phased arrayantenna receiver, in an RF mixer, and the like.

The foregoing has outlined some of the more pertinent features of thesubject matter. These features should be construed to be merelyillustrative. Many other beneficial results can be attained by applyingthe disclosed subject matter in a different manner or by modifying thesubject matter as will be described.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the subject matter and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a communications device that may be configured to use theapparatus of this disclosure;

FIG. 2 depicts the transceiver circuit of the communication device ofFIG. 1;

FIG. 3 depicts the RF up-converter and interference cancellation filtercircuit in the transceiver circuit of FIG. 2;

FIG. 4 depicts additional details of the optical FIR filter assembly ofthe filter circuit shown in FIG. 3;

FIG. 5 depicts an optical FIR filter and associated photodetectorarrangement wherein waveguide outputs are combined off-chip;

FIG. 6 depicts a representation of a photonic integrated circuit that isconfigured with an optical FIR filter assembly, and a set of convergingwaveguides according to the techniques of this disclosure;

FIG. 7 depicts a Focus Integrated Combined Output (FICO) apparatus ofthis disclosure comprising converging waveguides that enables efficienttransfer to an associated photodetector for off-chip conversion to theRF domain;

FIG. 8 depicts the FICO apparatus and an associated photodetector inadditional detail, illustrating how constructive and destructive fringesare captured and converted to the RF domain by the photodetector, allwithout losing optical power;

FIG. 9A depicts a prior art waveguide array configuration;

FIG. 9B depicts a waveguide array configuration of this disclosure inone embodiment;

FIG. 9C depicts a variant of the waveguide array configuration of FIG.9B that further includes isolation components to limit intermodulationproducts;

FIG. 10 depicts another variant waveguide array configuration thatincludes waveguide sizes and locations engineered to control an outputmode and/or divergence angle of the overall waveguide output from theFICO structure of this disclosure;

FIG. 11 depicts a further variant embodiment wherein phase shifterelements are positioned before one or more of the waveguide elements inthe FICO structure to enable engineering of the mode of light as ittransitions through the structure;

FIG. 12 depicts a phased array antenna receiver that includes a PIChaving a FICO structure of this disclosure;

FIG. 13 depicts a RF mixer that includes a PIC having a FICO structureof this disclosure; and

FIG. 14 depicts a representation of a FICO structure havingthree-dimensional (3D) waveguide array according to another embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts a communications device 100 including self-interferencecancellation capability, and in which the techniques of this disclosuremay be implemented. The communications device 100 includes a transceivercircuit 102, a processor 109, e.g., a central processing unit (CPU), amemory 106, and an assembly of modules 118, e.g., assembly of hardwaremodules, e.g., circuits, coupled together via a bus 108, over which thevarious elements 102, 109, 106 may communicate data and information.Memory 106 includes a communications routine 110 configured to controlcommunications operations for the communications device 100 includingcontrolling operation of the transceiver circuit 102, a control routine111, an assembly of modules 113, e.g., an assembly of software modules,and data/information 114. Data/information includes device information119, includes interface information including optical filter componentinformation and antenna information, etc., and communicationsdata/information 120 includes, e.g., RF frequency information, channeltype information, channel conditions, determined filter coefficients,received signal information, transmitted signal information, generatedradio frequency interference cancellation signal information, etc. Insome embodiments, some information stored in memory 106 is also storedin local memory within transceiver circuit 102. In some embodiments,processor 102, e.g., a CPU, executes routines including software modulesincluded in memory 106 to control the communications device 100 tocontrol the transceiver circuit 102 to implement a radio frequencyinterference cancellation method that includes the use of an opticalfilter assembly.

Transceiver circuit 102 includes a bus interface 107 and acommunications interface 113. Bus interface 107 couples the transceivercircuit to bus 108. Communications interface 113 couples the transceivercircuit 102 to one or more or all of: an antenna assembly 101, awaveguide 115 and a wire/cable 117. In some embodiments, the antennaassembly is included as part of the communications device 100. Antennaassembly 101 includes one or more antennas (103, . . . 106). In someembodiments, antenna assembly 101 includes a single antenna 103 which isused by both the transmitter and receiver of the transceiver circuit102. In some embodiments, that antenna assembly 101 includes a transmitantenna 103 and a receive antenna 106. In some embodiments, the antennaassembly includes a plurality of transmit antennas and a plurality ofreceive antennas. In some such embodiments, the antenna assembly 101 andthe transceiver circuit 102 support multiple-input multiple-output(MIMO) operations.

FIG. 2 illustrates the transceiver circuit 102 of FIG. 1 Transceivercircuit 102″ includes communications interface 113″, bus interface 107,transmit (TX) digital baseband (BB) circuit 206, TX digital baseband toanalog baseband circuit 204, TX analog baseband to radio frequency (RF)circuit 202, coupler device 226, signal combiner/coupler device 209, RXRF to analog baseband circuit 210, RX analog baseband to digitalbaseband circuit 212, RX digital baseband circuit 214, RF up-converterand interference cancellation filter circuit 222, and channel estimator,filter, e.g., digital filter, and filter control circuit 216, coupledtogether as shown. Signal combiner 209 is configured to combine areceived radio frequency signal 233 with the radio frequencyinterference cancellation signal 224 to produce a recovered radiofrequency signal 235. In various embodiments, the signal combiner 209 isconfigured to subtract the radio frequency interference signal 233 fromthe received radio frequency signal 233 to generate the recovered radiofrequency signal 235.

The transceiver circuit 102″ comprises of a transmit chain and thereceive chain. In the transmit chain, the transmit digital basebandcircuit 206 receives, via bus interface 107, input data 207 to betransmitted in the form of bits, converts the bits into a digitalbaseband waveform 205, which is output to the TX digital baseband toanalog baseband circuit 204. The TX digital baseband circuit 206performs encoding and modulation of the received input data 207. Theencoding and modulation performed by TX digital baseband circuit 106uses, e.g. orthogonal frequency division multiplexing, CDMA, or anotherencoding and modulation scheme. The TX digital baseband to analogbaseband circuit 204, e.g., a filter and digital to analog converter(DAC) assembly, converts the digital signal 205 into analog basebandsignal 203, which is output to TX analog baseband to RF circuit 202.Analog baseband signal 203 is received by TX analog baseband to RFcircuit 202 and subsequently up-converted to the operating RF frequencyusing a direct conversion or an intermediate frequency converterincluded in circuit 202. The up-converted RF signal 201 is the output ofa power amplifier included in circuit 202. The up-converted RF signal201 is coupled or divided using a device 226 where the pass-throughsignal 227 goes to the communication interface 113″ and the tappedsignal 223 is fed to the RF up-converter and interference cancellationfilter circuit 222. The RF signal 227 in the communication interface113″ passes through to the antenna 229 in case of this realization.

Receive antenna 231 receives a wireless RF signal and outputs receivedsignal 233 into in to interface 113″ toward the receive chain. On thereceive side of the transceiver circuit 102″, the receive signal 233from the communication interface 113″ feeds in to a coupler or combiner209 which is 3 port device. Coupler or combiner 209 is responsible forcombining input signal 224, which is an output of the RF-up converterand interference cancellation filter circuit 222, and input signal 233,which is the signal received via receive antenna 213, to generate outputRF signal 235. The output RF signal 235 is fed into the RX RF to analogbaseband circuit, 210, which is an RF down-converter, that down-convertsthe RF signal 235 into a baseband analog signal 211. This basebandanalog signal 211 is received, filtered and sampled by RX analogbaseband to digital baseband circuit 212, which generates and outputssampled output signal 213. The sampled output signal 213 is fed into theRX digital baseband circuit 214 including a digital receive processorthat is responsible for demodulation and decoding.

RF Signal 223, a copy of the transmit signal 201, is fed into the RFup-converter and interference cancellation filter circuit 222. RFup-converter and interference cancellation filter circuit 222 producessignal 224, which is a negative copy or near negative copy of theinterference signal received as a component of receive signal 233, theinterference signal being an effect of transmission of signal 227. Thecombining of the negative copy 224 with the received signal 233 using acombiner/coupler device 209 results in cancellation of interference thatis caused by the transmitter of transceiver circuit 102″ at the receiverof transceiver circuit 102″.

Channel estimator, filter and filter control circuit 216 interfaces withthe digital processing block of transmit digital baseband circuit 206and with the digital processing block of receive digital basebandcircuit 214. The channel estimator, filter, and filter control circuit216 is responsible for reconstruction of a residual interference signalthat is observed at the sampled signal 220 in the RX digital basebandcircuit 214. In particular, the channel estimator, filter and filtercontrol circuit 216 is responsible for the measurement and training of adigital filter included in circuit 216 and the RF cancellation filterincluded in circuit 222. To this end, channel estimator, filter, andfilter control circuit 216 uses input signal 219, a copy of the digitaltransmit signal, and received sampled signal 220 to determine the effectof the transceiver circuit 102″ and antennas (229, 231), determine thechannel that causes interference, and determine the appropriatecoefficients to be programmed to the RF interference cancellation filterincluded in circuit 222. The determined appropriate coefficients arecommunicated in signal 217 from channel estimator, filter and filtercontrol circuit 216 to RF up-converter and interference cancellationfilter circuit 222. Channel estimator, filter and filter control circuit216 also recreates a negative copy 221 of the interference signal, whichit sends to RX digital baseband circuit 214 to be subtracted from thereceived signal 213. RX digital baseband circuit 214 receives therecreated negative copy 221 of the interference signal and subtracts therecreated negative copy 221 of the interference signal from receivedsignal 213, as part of its processing. Circuit 214 further generatesdigital data out signal 215 and outputs digital data out signal viainterface 107.

FIG. 3 depicts a simplified representation of an RF up-converter andinterference cancellation filter circuit 222 of FIG. 2. RF up-converterand interference cancellation filter circuit 222″ includes a laser 302,an RF signal-to-optical signal converter 304 including a Mach-Zehndermodulator (MZM) 305, an optical filter assembly 306, and anoptical-to-RF converter 308, e.g., a balanced photo-detector, coupledtogether as shown. In a variant embodiment, laser 302 and RF-to-opticalconverter 304 are replaced by a directly modulated laser (DML). Radiofrequency signal-to-optical signal converter 304 has a radio frequencyinput 399 configured to receive a radio frequency signal, and an opticaloutput 397 for outputting a first optical signal generated from theradio frequency signal to be communicated. Optical filter assembly 306filters the first optical signal. Optical to radio frequency converter306 is coupled to an output of the optical filter assembly 306. Theoptical to radio frequency converter 306 is configured to generate aradio frequency interference cancellation signal from optical signalsoutput by the optical filter assembly 306. In various embodiments, andas mentioned, the optical to radio frequency converter 306 is a balancedphotodetector, and the optical to radio frequency converter 306generates the radio interference signal from an optical signal 391 andan optical signal 392 output from the optical filter assembly 306.

Optical filter assembly 306 typically includes an optical IIR filterassembly (not shown), an optical FIR filter assembly 313, a filtercontroller (not shown), a 1 to 2 optical coupler 310, two (2) 1 to Noptical couplers (312, 314) and two (2) N to 1 optical couplers (340,342) coupled together as shown in FIG. 3. As will be described, theoptical FIR filter assembly 313 typically includes a plurality ofoptical FIR filters, each optical FIR filter including a controllableoptical delay element, e.g., a delay device, and a controllable gainelement.

Laser 302 generates and outputs optical signal 326, which is sent to RFto optical converter 304, which receives that optical signal. TheRF-to-optical converter also 304 receives an input RF signal on radiofrequency input 399 and generates output optical signal 328, which isoutput on optical output 397. The optical signal is provided as an inputto the optical filter assembly 306. This optical signal is processed bythe optical IIR filter assembly, which generates and outputs an opticalsignal. The optical IIR filter assembly subjects the input opticalsignal to delays in accordance with the fixed delays corresponding tothe FRRs, and to gain adjustments in accordance with the controlled gainadjustments in accordance with the controlled gain settings of gaincontrol elements, thereby generating an optical output signal. Theoptical signal output from the optical IIR filter assembly is input tothe coupler 310, e.g., a 1 to 2 splitter, which generates first andsecond optical signals. The first optical signal is input to the 1×Ncoupler (splitter) 312, which outputs optical signals to the input(s) ofcontrollable delay devices (D11, D21, . . . DN1), respectively. Thesecond optical signal is input to the 1×N coupler (splitter) 314, whichoutputs optical signals to the input(s) of controllable delay devices(D12, D22, . . . , DN2), respectively. The optical delay devicesintroduce variable delays corresponding to the controlled delay settingssupplied by the filter controller, generating optical signals that arethen supplied as inputs(s) to controllable gain elements (AF11, AF21, .. . , AFN1, AF12, AF22, . . . , AFN2), respectively. The controllablegain elements adjust gains corresponding to the controlled gainssettings based on the filter control signals supplied by the filtercontroller, thereby generating resulting optical signals. Some of theseoptical signals are input to N×1 optical coupler (combiner) 340, whichcombines the signals and outputs an optical signal. The remainingoptical signals are input to N×1 optical coupler (combiner) 342, whichcombines the signals and outputs another optical signal. These opticalsignals output from the optical filter assembly 306 couple the opticalfilter assembly 306 to the optical-to-RF converter 304, in thisembodiment the balanced photodetector. Optical-to-RF converter 306receives the optical signals and generates and outputs an RF signal.

Thus, in the embodiment shown in FIG. 3, the RF signal is up-convertedand modulated onto an optical carrier, and the RF optical signal issplit into n taps, each of which goes thru a variable delay andgain/attenuation stage. The taps are combined resulting in a filteredsignal, which is converted back to RF using a photodiode (PD).

In practice, however, both the 1×N coupler and the N×1 coupler can bechallenging to physically realize. To see why this is so, reference isnow made to FIG. 4, which depicts additional details of the optical FIRfilter assembly 400. In this example embodiment, the drawing depicts thetop half (namely, coupler 312, the delay (D) and gain (AF) elements, andcoupler 340 of the optical FIR filter assembly shown in FIG. 3). Thisstage is implemented in a tree-based configuration on the PIC. Asdepicted, the input RF optical signal is split into multiple taps usingadjustable power splitters 402, with the output of splitter feeding theinput to another. Each tap passes thru a delay (D) and gain (AF)element, the taps are combined in pairs by a power combiner 404, withthe output of a particular power combiner feeding to another, etc. Asnoted above, this structural configuration is difficult to realize. Inparticular, when the taps are combined (by the power combiner elements)energy is lost due to destructive interference due to the phasedifference between the combining waveguide. Although the lengths of thewaveguides are known, it is not possible to accurately predict thesephases and force a coherent combination as the gain stage changes thephase of the signal based on the gain. As a result, this topologyresults in significant lost power, and it is not readily physicallyrealizable.

One possible approach to address this problem is depicted in FIG. 5. Inthis example, the optical FIR filter assembly 500 comprises the 1×Ncoupler 502 and the plurality of delay (D) and gain (AF) elements asbefore, but in this case the outputs are taken off the PIC into multiplephotodetector (PD) elements 504. The outputs of elements 504 are thencombined off-PIC and in the RF domain by the N×1 coupler 506. Thisapproach, however, is difficult to implement from a chip packaging andassembly standpoint. Moreover, the approach requires significant addedcomplexity and cost, and is less reliable that performing the combiningon-chip, in part because the RF combiners typically contributesignificant signal loss.

Optical circuits having waveguide arrays can provide different modes oflight transmission. These include single mode, multi-mode andsuper-mode. In single mode, an individual waveguide supports just onemode of propagation. In multi-mode, an individual waveguide supportsmore than one mode of propagation. Super-mode refers to a guided modethat exists between multiple discrete waveguides that are configured insingle or multi-mode.

According to the approach herein, and in lieu of taking waveguideoutputs off a PIC for conversion to RF and combining in the RF domain(FIG. 4), the waveguides are configured in a waveguide array thatprovides significant efficiencies, thereby enabling the couplers to bephysically-realized on the PIC. As depicted in FIG. 6, a photonicintegrated circuit (PIC) 600 is configured with various elementscomprising, among others, an optical FIR filter assembly such asdepicted in FIG. 3. A waveguide array is fabricated on (e.g., patternedonto, placed on top of, etc.) the integrated photonic chip. Inparticular, and in this embodiment, the individual waveguides asfabricated are configured to converge very close to one another at anexit 602 of the PIC. The exit 602 preferably is located along an edge604 of the chip. The particular location of the exit is not alimitation. FIG. 7 depicts this structural arrangement (an array ofconverged waveguides) in additional detail. This arrangement issometimes referred to herein as a Focus Integrated Combined Output(FICO) structure or apparatus.

In this example embodiment, the waveguide array comprises two (2) setsof waveguides depicted as being formed on (etched into) the substrate700 (which may be formed by leveraging conventional patterningtechniques), with a first set 702 corresponding to the outputs of theN×1 combiner (e.g., coupler 340 in FIG. 3), and the second set 704corresponding to the outputs of the other N×1 combiner (e.g., coupler342 in FIG. 3). Each such set is a FICO structure. As depicted in FIG.7, which is merely exemplary, FICO structure is two-dimensional (2D),and the respective waveguides therein are positioned with respect to acentral axis (of the set), with individual waveguides of the set taperedsuch that the waveguide outlets (for the set) converge very close to oneanother at the exit of the chip. There is no requirement that thewaveguides be centered about a particular central axis; in analternative 2D arrangement the waveguides forming the FICO structure maybe co-planar but offset (e.g., slanted) relative to a particular planeof the chip. The FICO structure also may be configured inthree-dimensions (3D), as further explained below, or as a consequenceof individual waveguides being merely offset from one another (i.e., notco-planar) along some portions thereof.

Thus, according to this disclosure, at least a first waveguide in a setof waveguides is patterned on the optical circuit at least in part in aconverging configuration such that, relative to at least one secondwaveguide in the set, the resulting outlets of the first and secondwaveguides are spaced in a manner to facilitate the generation orinhibition (from the array) of intermodulation products and/orharmonics. Preferably, the first and second waveguides are configuredrelative to one another such that light entering both waveguides (evenat the same polarization) propagates through portions of the waveguidesthat are positioned sufficiently adjacent to one another (and that thentypically terminate along a same PIC facet); this type of propagation(across nearby portions of the waveguides) is sometimes referred toherein as co-propagation and can excite a supermode between thewaveguides. In a typical case, co-propagation occurs over a length orextent of the adjacent waveguide portions, and the amount of suchco-propagation (e.g., corresponding to the common length portions) thendetermines a degree to which intermodulation products and/or ‘arecreated or limited. The adjacent waveguide portions are sometimesreferred to herein as a co-propagation region.

As a concrete example, FIG. 7 depicts a FICO structure with relativelylimited overlap of the converged waveguide sections before thetermination along the common facet; in this example, the intermodulationproducts are relatively limited/restricted. On the other hand, and asdepicted in FIG. 8 (described below), when the converged waveguideelements are co-extensive over a much longer extent (and thus have alonger co-propagation region), more intermodulation products aregenerated.

As these examples evidence, the amount and nature of the intermodulationproducts can thus be selectively tuned by adjusting the structuralarrangement of the converged waveguide(s) that, typically, terminate atthe same PIC facet. Stated another way, and given first and secondwaveguides (with one converging toward the other at least in part), thelight enters both waveguide inlets, and it transits the waveguides. Dueto the converging structure, the light also co-propagates along theportions of the first and second waveguides that lead to the terminus,thereby creating/inhibiting intermodulation products and/or harmonics asa function of the co-propagation (typically, length); the light, havingnow passed through both waveguides, then terminates from the array atthe common facet. The nature and extent of the intermodulation productsor harmonics produced by the waveguides thus are tunable by controllingthe co-propagation length. In an alternative, co-propagation may beimpacted by other structural arrangements or relationships of thewaveguide portions (e.g., orientation, size, shape, input phase of theoptical signal input to the respective guides, etc.)

In addition, the properties of the co-propagation region and thus theintermodulation products or harmonics generated or inhibited may changedue to environmental factors, such as temperature, pressure, stress,vibration and the presence of gasses or other substances. In one examplescenario, one or more of the following factors thus may be engineered togenerate a desired output from the FICO device: length ofco-propagation, phase of one or more individual waveguides, intensity ofone or more of the individual waveguides, temperature of the system ordevice (which in effect alters co-propagation length), properties of thewaveguides and separation material (which may be altered duringfabrication using temperature or temperature gradients). More generally,if the impact of such factors are known or ascertainable (andcontrollable), it is within the scope of this disclosure to tune theco-propagation region further to take advantage of one or more suchfactors.

The above-described structural arrangement (e.g., the co-propagationregion) facilitates the focused integration of the waveguide outputs,hence the FICO name. As a skilled person will appreciate, the generationor inhibition of intermodulation products and/or harmonics, as the casemay be, each represent different forms of distortion of the modulatedsignal produced by the FICO structure.

In the example embodiment in FIG. 7, the resulting waveguide of the FICOstructure has an aperture that is sized to be close to the size of thephotodetector. In particular, FIG. 8 depicts in detail one of the setsat the very edge of the chip, as well as a photodetector (PD) 800, whichis positioned off-chip but nearby (adjacent to the edge). As shown, theaperture 805 of the resulting waveguide (comprising the apertures of theindividual waveguide elements 802 converged around the central axis 804)is close to the size of the PD. Thus, in this arrangement, a FICOstructure (such as first set 702, or second set 704 in FIG. 7)comprising a set of waveguides having individual apertures and that areconfigured so as to act as a single, larger, aperture. As configured,and as depicted on FIG. 8, in the typical use case, the individualwaveguide outputs from the set are combined on the photodetector 800.

Although FIG. 8 depicts the FICO structure as having an aperture that isclose to the size of the photodetector, this is not a requirement. Theaperture of the waveguide array may be smaller (or even larger) than thesize of the photodetector without departing from the principles of thisdisclosure. Generalizing, the positioning of the photodetector relativeto the chip edge, and the size of waveguide array aperture relative tothe photodetector size, may vary depending on the desired application oruse case.

The above-described structural arrangement provides significantadvantages. In particular, the approach allows all of the energy fromthe individual waveguides to be transferred to the PD, which energy isthen converted to the RF domain for further processing (in thisembodiment). In particular, and by sizing the aperture close to that ofthe PD, constructive and destructive fringes are captured and converted,thereby avoiding optical power (interference-related) losses.

Although the waveguide array is depicted in FIGS. 7 and 8 as being two(2)-dimensional, this is not a limitation, as three (3)-dimensional (3D)structures may also be utilized. Thus, in general there is norequirement that the waveguides comprising the FICO structure befabricated in the same plane of the chip. A representation of a 3Dwaveguide structure is shown in FIG. 14. A three-dimensional (3D) FICOstructure of this type allows for the creation of multiple modes(“multi-mode”) simultaneously, as well as the coupling of such multiplemodes (e.g., with multiple information signals) into awaveguide/PIC/optical fiber. As depicted in FIG. 14, the waveguides inthe 3D structure may have different sizes, shapes, and orientationrelative to one another. A 3D structure of this type also enablescreation of a phased array optical interface, thereby allowing beamsteering and efficient coupling to diverse targets.

Referring now to FIGS. 9A-9C, the mode of transmission that is enabledby the structural arrangement of the converged waveguides in thewaveguide array is now shown and compared to the prior art. FIG. 9A,which depicts the prior art, shows a conventional transmission mode forthe waveguides 900, 902 and 904. As can be seen, in this arrangement thephotons 905 in a particular waveguide (e.g., waveguide 900) do notinteract with the photons in an adjacent waveguide (e.g., 902) due tothe physical separation of the waveguides in the array. In FIG. 9B,however, and in contrast to the arrangement shown in FIG. 9A, thewaveguides 901, 903 and 905 are configured in the converged mannerpreviously described and thus are closely packed with respect toone-another; in this arrangement, the photons instead interact acrossthe closely-spaced waveguides to form a super-mode 907 of transmissionthat encompasses the waveguides in the set. The photons from theindividual waveguides, however, do not interfere with one another. In avariant embodiment, as depicted in FIG. 9C, individual waveguides in theset may be isolated from one another, and thereby limiting generation ofintermodulation products, e.g., by interposing an isolation trench(e.g., 909) between individual waveguides. The isolation trench maycomprise any suitable material having a low refractive index. In oneexample, the trench may comprise an air trench. Isolating individualwaveguides in this manner is not required, but this structuralarrangement may be useful when intermodulation products are desired tobe limited.

Generalizing, the FICO apparatus of this disclosure comprises an arrayof waveguides patterned onto a PIC. The array comprises individualwaveguides, each of which receive light inputs, e.g., individual taps ofa multi-tap optical filter used in an interference cancellation circuit.Each individual waveguide comprises an inlet, and an outlet; typically,the output(s) of the individual waveguides are located at an exit (edge)of the PIC. Typically, the waveguide array as structured is configuredalong an axis passing through a first waveguide. According to thisdisclosure, at least one second waveguide in the array is patterned onthe PIC in a tapered configuration such that, relative to the firstwaveguide, the outlet(s) of the first and second waveguides arepositioned to be closer to one another as compared to the inlet(s) ofthese waveguides. This structural configuration enables the generationof the super-mode of transmission at the PIC exit, enabling moreefficient transfer of the energy, e.g., to an associated PD thatprovides conversion of the energy to the RF domain.

Further, it is not required that the individual waveguides in thewaveguide array set have the same physical structure (namely, width,length, etc.). Referring now to FIG. 10, the output mode and divergenceangle of the composite waveguide output may be varied by engineering thesize and shape of individual waveguides in the FICO structure. Thisallows optimization of the number of taps at the exit, the size of thephotodetector, and the distance of the photodetector from the PIC edge.Thus, in the example arrangement shown in FIG. 10, the waveguide arrayfor the FICO structure comprises waveguides 1002, 1004, 1006, 1008 and1010. As depicted, the waveguides 1004, 1006 and 1008 have smallerwidths than waveguides 1002 and 1010. In this arrangement, the resultingmode 1012 is narrower than the overall width of all the waveguides takentogether, thereby allowing for more taps or a smaller photodetector.Output modes that are engineered in this way, e.g., by adjusting therelative sizes and spacing of the waveguides, is very desirable as itenables effective (efficient) coupling of the FICO structure output toother waveguides/PICs/optical fiber. The approach provides productionefficiencies as the transmission mode may also be changedpost-fabrication.

Generalizing, the number of waveguides (taps) in the FICO structure mayvary, as can their shape(s) and relative spacing. Where desirable, andas also described, waveguides may be further separated by isolationelements to limit intermodulation products. Further, individualwaveguides may include associated phase shifter elements at theirleading ends (i.e. their inlets) for further signal-shaping.

In accordance with another variant embodiment, a phase shifter isassociated with one or more of the individual waveguides that arepatterned on the PIC. The phase shifter(s) preferably are located at afront end of the waveguide array (the FICO structure), which in FIG. 8(for example) is the location to the left of the array. Inclusion ofphase shifters in this manner enables the mode of light to be furtherengineered as it transitions through the waveguide array structure. Anexemplary arrangement is depicted in the block diagram shown in FIG. 11.In this example, the optical FIR filter assembly 1100 comprises the 1×Ncoupler 1102, delay elements 1104, gain elements 1106, and the N×1combiner 1110 that comprises the waveguide array structure configured tocombine the multiple paths of light as previously described. As alsodepicted in FIG. 11, and in this variant embodiment, a set of phaseshifters 1108 also are configured on the chip, with each phase shifter1108 element located in front of a respective waveguide in the arraythat forms the combiner 1110. In this example, there is a phase shifterconfigured in front of each waveguide, although this is not arequirement.

While the FICO structure provides significant advantages when used as anoptical filter in association with a photodetector as previouslydescribed, this is not a limitation. The following describes various usecases that benefit from this subject matter.

In one example, the output provided by the converged waveguides in theFICO structure are coupled as an input to another photonic chip in lieuof being converted to the RF domain for further processing. In thisarrangement, the photodetector is not used.

In another example, the FICO structure facilitates creation of a phasedarray optical interface, allowing beam steering and efficient couplingto diverse targets.

In still another variant, depicted in FIG. 12, the FICO structure isused in a phased array antenna receiver 1200. As shown, the receiveronly requires use of a single laser 1201, and a single photodetector(PD) 1203. As depicted, the output of the laser 1201 is split by aplurality of power splitters 1205. The phased array antenna receiver1200 comprises multiple receive antennas 1202, with the signal outputfrom each antenna then being processed through a processing leg thatincludes an RF receive signal chain 1207. In this configuration, thesignals from the multiple receive antennas 1202 are modulated onto anoptical carrier, using an RF-to-optical converter or modulator 1212 ineach leg. The optical versions of the receive signals are coupled onto aPIC 1204 where each goes through a variable delay 1206 stage andvariable gain/attenuation stage 1208. The signals from each leg are thencombined using a FICO structure 1210 as they exit the PIC and areconverter back to the RF domain by the PD 1203. A phased array may haveas few as two elements (antennas) or as many as n elements. In anothervariant of this configuration, it may be desirable to support multiplefiltered signals (e.g., corresponding to multiple spatial beams), andthis can be accomplished by using multiple layers of the PIC filter orby modulating the received signal onto multiple wavelengths of light.

In a phased array antenna of this type, it is important to accuratelyadjust the phase/delay of multiple received signals from multipleantennas to reconstruct a given spatial beam. Using a PIC, high-fidelityswitched, true time delay lines are implemented, and these variabledelays are used to accurately reconstruct a given spatial beam. The FICOstructure facilitates recombining the signals to recover the spatialbeam, as well as to facilitate efficient coupling to the photodetectorefficient optical-to-electrical conversion.

FIG. 13 depicts another use case, an RF mixer using a PIC thatincorporates the FICO structure described herein. In this example, alaser 1302 output is provided to a power splitter 1304, which in turndrives a pair of RF-to-optical converters 1306 and 1308. The convertersoutput optical signals to an optical filter and mixer assembly 1310patterned on the PIC. The assembly 1310 comprises a filtering stage1312, and the FICO structure 1314, with the latter used to mix thefiltered signals provided by the filtering stage 1312. The filteredsignal is converted back to the RF domain using a PD 1316.

Enabling Technologies

The FICO apparatus of this disclosure is configured to be fabricated ona photonic integrated circuit. Generalizing, a PIC (or, more generally,an integrated optical circuit) is a device that integrates a pluralityof photonic functions, typically functions for information signalsimposed on optical wavelengths typically in the visible spectrum or nearinfrared 850 nm-1650 nm. Several of these functions have been describedherein by way of example, but these examples are not intended to belimiting. Photonic integrated circuits are fabricated from a variety ofmaterial systems, including electro-optic crystals such as lithiumniobate, silica on silicon, Silicon on insulator, as well as variouspolymers and semiconductor materials. According to this disclosure, aphotonic integrated circuit is configured to include at least one FICOapparatus configured as has been described and depicted.

The description above concerning a waveguide “inlet” or “outlet” is notintended to be limiting. Depending on the use case, a particular inletof a waveguide may function as an outlet, or vice versa.

What is claimed is:
 1. A waveguide structure for optically coupling afirst tap output of an optical filter assembly to a photodetector and asecond tap output of the optical filter assembly to the photodetector,the waveguide structure comprising: a substrate defined by a length; afirst optical waveguide disposed at least partially on the substrate andcomprising: a first inlet optically coupled to the first tap output; afirst linear region extending at least partially across the length ofthe substrate; and a first transition region optically coupling thefirst linear region to a common output optically coupled to thephotodetector; and a second optical waveguide disposed at leastpartially on the substrate and comprising: a second inlet opticallycoupled to the second tap output; a second linear region extending atleast partially across the length of the substrate parallel to the firstlinear region of the first optical waveguide; and a second transitionregion converging toward the first transition region, the second opticalwaveguide optically coupling the second linear region to the commonoutput; wherein: a distance separating the first optical waveguide fromthe second optical waveguide at a location along the length of thesubstrate facilitates a supermode of photons traversing the firstoptical waveguide and a supermode of photons traversing the secondoptical waveguide.
 2. The waveguide structure of claim 1, wherein thefirst optical waveguide is formed at least partially onto the substrate.3. The waveguide structure of claim 1, further comprising an isolationtrench separating at least a portion of the first optical waveguide fromat least a portion of the second optical waveguide.
 4. The waveguidestructure of claim 3, wherein the isolation trench is formed from amaterial having a low refractive index.
 5. The waveguide structure ofclaim 4, wherein the isolation trench is an air trench.
 6. The waveguidestructure of claim 1, wherein the location is within the firsttransition region.
 7. The waveguide structure of claim 1, wherein thesecond transition region overlaps at least a portion of the first linearregion along the length of the substrate.
 8. The waveguide structure ofclaim 1, wherein the substrate forms at least a portion of a photonicintegrated circuit.
 9. The waveguide structure of claim 1, wherein thefirst optical waveguide is coplanar with the second optical waveguideacross the length of the substrate.
 10. An on-chip structure foroptically coupling an array of optical outputs of an optical filterassembly to a photodetector, the on-chip structure comprising: a set ofoptical waveguides comprising: a set of inlets physically separated fromone another, each inlet optically coupled to a respective one opticaloutput of the optical filter assembly; and a common outlet, the commonoutlet optically coupled to each inlet of the set of inlets, the commonoutput optically coupled to the photodetector; wherein: a first opticalwaveguide of the set of optical waveguides has a first rectangularcross- sectional shape and a second optical waveguide of the set ofoptical waveguides has a second rectangular cross-sectional shape. 11.The on-chip structure of claim 10, wherein each optical waveguide of theset of optical waveguides has a rectangular cross-sectional shape. 12.The on-chip structure of claim 10, wherein the first rectangularcross-sectional shape is different from the second rectangularcross-sectional shape.
 13. The on-chip structure of claim 10, furthercomprising a set of isolation trenches disposed between each of the setof optical waveguides.
 14. The on-chip structure of claim 10, whereinthe set of optical waveguides are formed onto a substrate of a photonicsintegrated circuit.
 15. An on-chip structure for optical coupling, theon-chip structure comprising: a substrate defined by a first length; anda first optical waveguide defining a first inlet and defined by a secondlength; a second optical waveguide defining a second inlet and definedby a third length; and a common output optically coupled to each of thefirst inlet and the second inlet; wherein: the first length is less thanthe second length and the second length is less than the third length;and a distance separating the first optical waveguide and the secondoptical waveguide at a location along the length of the substratefacilitates a supermode of photons traversing the first optical.
 16. Theon-chip structure of claim 15, wherein the first inlet and the secondinlet are separated by a distance.
 17. The on-chip structure of claim15, wherein at least a portion of the first optical waveguide isoptically isolated from the second optical waveguide.
 18. The on-chipstructure of claim 15, wherein the common output is optically coupled toa photodetector.